The present invention relates to a lenticular lens sheet that is suitably used in combination with a Fresnel lens sheet and to a projection screen that is equipped with this lenticular lens sheet.
As illustrated in FIG. 4, in a projection screen on the light-incidence surface side and on the light-emission surface side, respectively, there are disposed a Fresnel lens sheet F and a lenticular lens sheet L. The Fresnel lens sheet is the one that has a base sheet having formed thereon a Fresnel lens by the use of a UV resin. For preventing the generation of a stray light (double image), the thickness thereof is made as small as 0.2 to 3.0 mm or so.
On the other hand, the lenticular lens sheet is a sheet the thickness of that is approximately 0.6 to 1. 1 mm and on the surface of that a lenticular lens has been formed using an extrusion-molding technique.
The screen for that there is used a lens sheet having a relatively small thickness usually has a size as great as 40 to 80 inches. Therefore, between the lenticular lens sheet and the Fresnel lens sheet a separation, or wrinkles or undulations, are likely to occur. To prevent such phenomena, the following measures have been proposed. Namely, the measures to form the lenticular lens sheet into a configuration that is convex to the light-incidence surface side (Fresnel lens sheet side) (see FIG. 5(a)). And to bond it and the Fresnel lens sheet side together so as to cause the both to adhere to each other (see FIG. 5(b)). Japanese Patent Application Laid-Open No. 2000-235230 discloses a construction for realizing the above-described measures by using acrylic resin with high hygroscopic on the light-incidence surface side of the lenticular lens sheet.
However, humidity depends upon the surroundings of use of the projection screen. Therefore, there has been a demand for the settlement of the above-described problems in a stage before the stage in which such use is made.
Thereupon, it is the object of the present invention to provide a lenticular lens sheet that in the stage of manufacture can solve the above-described problems and a projection screen that is equipped with that lenticular lens sheet.
A transparent plastic material such as acrylic resin, PET resin, polystyrene resin, or polypropylene resin or the like necessarily has a peculiar glass transition point temperature (a temperature that when becoming higher than this temperature causes the material to start being fluidized) for each kind of the material. The present invention has been achieved by aiming at the fact that with that glass transition point temperature being set as a border the coefficients of linear expansion at temperatures higher or lower than that temperature remarkably change.
Hereafter, the present invention will be explained.
In a first aspect of the present invention, the above object is attained by a composite lenticular lens sheet that is disposed on a light-emission surface side of a Fresnel lens sheet and on a light-incidence surface side and on the light-emission surface side of that there are used respectively different plastic materials, the composite lenticular lens sheet being characterized in that, when it is assumed that, regarding the light-emission surface side material, Ta(xc2x0 C.) represents the glass transition point; Am(1/xc2x0 C.) represents the coefficient of linear expansion at a temperature equal to or higher than Ta; and As(1/xc2x0 C.) represents the coefficient of linear expansion at a temperature equal to or lower than Ta, while, regarding the light-incidence surface side material, Tb(xc2x0 C.) represents the glass transition point; Bm(1/xc2x0 C.) represents the coefficient of linear expansion at a temperature equal to or higher than Tb; and Bs(1/xc2x0 C.) represents the coefficient of linear expansion at a temperature equal to or lower than Tb; and when it is also assumed that Tx(xc2x0 C.) represents a predetermined temperature equal to or higher than each of the Ta and Tb, the light-emission surface side and light-incidence surface side materials are selected so that the following relationship may hold true.
(Txxe2x88x92Ta)Am+(Taxe2x88x9225)As greater than (Txxe2x88x92Tb)Bm+(Tbxe2x88x9225)Bs 
Here, regarding the coefficients of linear expansion Am, As of the light-emission surface side material as well as regarding the coefficients of linear expansion Bm, Bs of the light-incidence surface side material, the relevant explanation will be made with the following setting being made with regard to each coefficient of linear expansion that has dependency uppon temperatures. Namely, the corresponding temperature range is equally divided into 10 points (excluding the point at each end). And, said temperature-dependency coefficient of linear expansion value uses the arithmetic average value of the respective coefficient of linear expansion values at those 10 equally divided points temperatures.
According to the present invention, the light-emission surface side material and light-incidence surface side material that have been extruded in liquid forms, during a time period in which cooling is performed from the temperature Tx to normal temperature (25xc2x0 C.), respectively, decrease (shrink) from the reference volume at the temperature Tx by the extent corresponding to amounts that are expressed as follows.
3((Txxe2x88x92Ta)Am+(Taxe2x88x9225)As) 
3((Txxe2x88x92Tb)Bm+(Tbxe2x88x9225)Bs) 
Here, the reason why the coefficient xe2x80x9c3xe2x80x9d is added to each of the foremost portions of the volume terms is as follows. Namely, a cubic body the one-side length of that is xe2x80x9c1xe2x80x9d is considered as a reference and when the coefficient of liner expansion is X. Assuming that the temperature has risen by 1xc2x0 C., the volume of this cubic body changes to (1+X)3=1+3X+3X2+X3. However, because the coefficient X of linear expansion is sufficiently smaller than the value of xe2x80x9c1xe2x80x9d, the term xe2x80x9c3X2+X3xe2x80x9d is a magnitude that is ignorable. Accordingly, the change in volume (the coefficient of volume expansion) relative to the change in temperature by 1xc2x0 C. becomes 3xc3x97. That is, it becomes a value that is equal to approximately 3 times as great as the coefficient of linear expansion.
Accordingly, by selecting the light-emission surface side material and the light-incidence surface side material so that the following enequality may hold true, it is possible to make great the amount of shrinkage of the light-emission surface side material relative to the light-incidence surface side material during cooling.
(Txxe2x88x92Ta)Am+(Taxe2x88x9225)As greater than (Txxe2x88x92Tb)Bm+(Tbxe2x88x9225)Bs 
It is thereby possible to make the whole configuration of the composite lenticular lens sheet convex to the light-incidence surface side.
In a second aspect of the present invention, the above-described Tx may be a temperature that is obtained by adding a predetermined temperature to the temperature of (Ta+Tb)/2. Also, the predetermined temperature may be 5 to 85xc2x0 C. Further, the Tx may be 150xc2x0 C.
In this aspect, the Tx can be set at a temperature that is somewhat higher than each of the real transition point temperatures of the two kinds of materials. Therefore, even if the dependency upon the temperatures of each of the coefficients of linear expansion of the materials is high, these coefficients of linear expansion are almost not affected by that dependency. And during a time period in which cooling is performed from the temperature Tx to 25xc2x0 C., it is possible to make great the amount of shrinkage of the light-emission surface side material relative to the light-incidence surface side material. Resultantly, it is possible to make the whole configuration of the composite lenticular lens sheet convex to the light-incidence surface side.
In a third aspect of the present invention, the above object is solved by providing a composite lenticular lens sheet that is characterized in that the light-emission surface side and light-incidence surface side materials are selected so that the following relationship may hold true.
(Tbxe2x88x92Ta)Am+(Taxe2x88x9225)As greater than (Tbxe2x88x9225)Bs 
If that composite lenticular lens sheet is provided like this, the light-emission surface side material and light-incidence surface side material that have been extruded in liquid states, during a time period in which cooling is performed from the temperature Tb to normal temperature (25xc2x0 C.), respectively, decrease (shrink) from the reference volume at the temperature Tb by the extent corresponding to amounts that are expressed by the following value.
3((Tbxe2x88x92Ta)Am+(Taxe2x88x9225)As) 
and
3(Tbxe2x88x9225)Bs 
Accordingly, by selecting the light-emission surface side and light-incidence surface side materials so that the following relationship:
(Tbxe2x88x92Ta)Am+(Taxe2x88x9225)As greater than (Tbxe2x88x9225)Bs 
may hold true, it is possible to make great the amount of shrinkage of the light-emission surface side material relative to the light-incidence surface side material. Resultantly, it is possible to make the whole configuration of the composite lenticular lens sheet convex to the light-incidence surface side.
Also, in a fourth aspect of the present invention, the above object is solved by providing a composite lenticular lens sheet characterized in that the light-emission surface side and light-incidence surface side materials are selected so that the following relationship may hold true.
Ta less than Tb 
Also, in each of the first to the third aspect of the present invention, the light-emission surface side and light-incidence surface side materials may be selected so that the following relationship may hold true.
Ta less than Tb 
If providing the composite lenticular lens sheet like this, in the process of cooling, the light-incidence surface side material is first solidified and then the light-emission surface side material is solidified and shrunken to draw the light incidence surface side material toward it. Resultantly, it is possible to make the whole configuration of the composite lenticular lens sheet convex to the light-incidence surface side.
Further, the above-explained composite lenticular lens sheet can be disposed on the light emission side of the Fresnel lens sheet to thereby cause the both to adhere to each other. By doing so, it is possible to provide a projection screen that can prevent any separations from occurring between the lenticular lens sheet and the Fresnel lens sheet or can prevent any wrinkles or undulations from occurring on the surface of the lens sheet.
The above-described functions and advantages of the present invention will become apparent from the embodiments that will be explained next.